The present invention is directed to a focusing ion lens system for mass spectrometer for separating charged from neutral particles.
Normally, a mass spectrometer is used in conjunction with an ionizer where the ions originate under low pressure conditions such as 10.sup.-3 torr. However, recently developed techniques of ionizing sample molecules involve operation of the ionizer at elevated pressures of, for example, greater than 10.sup.-2 torr. This is done in order to utilize the resultant collisions of sample molecules with ionized reagent gas molecules as a means of ionizing the sample molecules. Such operation will produce a mixture of positive and negative ions, electrons and neutral particles.
In order to separate the positive or negative ionic particles from the abundant neutral particles emanating from such a high pressure ion source and subsequently focus the resultant ion beam into a mass analyzer the following has been done in the prior art. Specifically, in most mass spectrometers equipped with a high pressure ionizer a differentially pumped vacuum system is employed so that the ionizer is pumped separately from the mass analyzer and detector. This arrangement allows a lower pressure to be maintained in the vicinity of the mass analyzer. A differential pumping baffle is maintained between the mass analyzer and the ionizing region. However, in order for the low pressure to be maintained in the mass analyzer the bulk of the gas leaving the ionizer must be pumped away by the ionizer pump. It is desirable to retain the ionic particles of interest and focus them into the mass analyzer. This has been accomplished in the past by narrowing the outlet of the ionizing region with subsequent loss of ionic species.